Semiconductor nanocrystals have been intensely investigated in recent years. The properties of these crystals differ from those of the bulk solids (Weller, Adv. Mater. 5:88 (1993); Stucky, Prog. Inorg. Chem. 40:99 (1992); Stucky and MacDougall, Science 247:669 (1990); Brus and Steigerwald, Acc. Chem. Res. 23:183 (1990); Siegel, Physics Today, p. 64 (October 1993)). For example, reduced melting temperatures of nanocrystals make possible low-temperature routes of thin-film formation (Siegel, Physics Today, p. 64 (October 1993); Goldstein et al, Science 256:1425 (1992)).
Synthesis of nanocrystalline semiconductors (or their precursors) has been effected using silyl cleavage. Specifically, Group III halides have been reacted with E(SiMe.sub.3).sub.3 (E=P, As) in hydrocarbon solvents to yield nanocrystalline III-V (13-15) semiconductors (or their precursors) (Wells et al, Chem. Mater. 1:4 (1989); Wells et al, Mater. Res. Soc. Symp. Proc. 131:45 (1989); Wells et al, Chem. Mater. 3:382 (1991); Wells et al, Organometallics 12:2832 (1993); Aubuchon et al, Chem. Mater. January (1994)). Synthesis of GaAs nanocrystals in decane has also been reported (Butler et al, J. Phys. Chem. 97:10750 (1993)). Silyl cleavage has also been used to synthesize GaAs nanocrystals in quinoline (Olshavsky et al, J. Am. Chem. Soc. 112:9438 (1990); Uchida et al, J. Phys. Chem. 95:5382 (1991)). In addition, the synthesis of GaAs and InAs nanocrystals from Ga(acac).sub.3 and In(acac).sub.3, respectively, by reactions with As(SiMe.sub.3).sub.3, has been described. However, the formation of byproducts and the fate of the acetylacetonate ligands were not reported (Uchida et al, J. Phys. Chem. 96:1156 (1992); Uchida et al, Chem. Mater. 5:716 (1993)).
Recently Kaner and co-workers reported a general method of synthesizing binary III-V semiconductors involving solid state metathesis (SSM). According to this method, sodium pnictides are reacted with Group III halides, either in bombs or sealed glass ampules, at high temperatures. These exothermic reactions generate enough heat to melt the sodium halide product. SSM reactions, therefore, often yield polycrystalline III-V semiconductors contaminated with starting materials and byproducts (Treece et al, Inorg. Chem. 32:2745 (1993); Treece et al, Mater. Res. Soc. Symp. Proc. 271:169 (1992); Treece et al, Chem. Mater. 4:9 (1992); Wiley and Kaner, Science 255:1093 (1992)).
An important aspect of Kaner's work involved controlling the particle size by adding inert materials as heat sinks to the SSM reaction mixtures. The particle size of MoS.sub.2, synthesized from SSM reaction between MoCl.sub.5 and Na.sub.2 S, was altered by the amount of NaCl added to the reactants. The greater the amount of NaCl, the smaller the particle size of MoS.sub.2 obtained (Wiley and Kaner, Science 255:1093 (1992)).
The present invention avoids the severe conditions and high temperatures of the SSM reactions that result in polycrystalline semiconductor powders contaminated with undesirable materials such as byproducts and unreacted starting materials. The present invention provides a low temperature, solution phase approach to the production of nanocrystalline III-V semiconductors that avoids the use of highly toxic and pyrophoric hydrides as well as pyrophoric and air-sensitive Group III alkyl compounds.